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Changes in Sea Level 11 Changes in Sea Level Co-ordinating Lead Authors J.A. Church, J.M. Gregory Lead Authors P. Huybrechts, M. Kuhn, K. Lambeck, M.T. Nhuan, D. Qin, P.L. Woodworth Contributing Authors O.A. Anisimov, F.O. Bryan, A. Cazenave, K.W. Dixon, B.B. Fitzharris, G.M. Flato, A. Ganopolski, V. Gornitz, J.A. Lowe, A. Noda, J.M. Oberhuber, S.P. O’Farrell, A. Ohmura, M. Oppenheimer, W.R. Peltier, S.C.B. Raper, C. Ritz, G.L. Russell, E. Schlosser, C.K. Shum, T.F. Stocker, R.J. Stouffer, R.S.W. van de Wal, R. Voss, E.C. Wiebe, M. Wild, D.J. Wingham, H.J. Zwally Review Editors B.C. Douglas, A. Ramirez Contents Executive Summary 641 11.3.2.3 Mean sea level change from satellite altimeter observations 663 11.1 Introduction 643 11.3.3 Changes in Extreme Sea Levels: Storm Surges and Waves 664 11.2 Factors Contributing to Sea Level Change 644 11.2.1 Ocean Processes 644 11.4 Can 20th Century Sea Level Changes be 11.2.1.1 Observational estimates of ocean Explained? 664 warming and ocean thermal expansion 644 11.5 Future Sea Level Changes 666 11.2.1.2 Models of thermal expansion 646 11.5.1 Global Average Sea Level Change 11.2.2 Glaciers and Ice Caps 647 1990 to 2100 666 11.2.2.1 Mass balance studies 647 11.5.1.1 Projections for a single scenario 11.2.2.2 Sensitivity to temperature change 647 based on a range of AOGCMs 666 11.2.2.3 Sensitivity to precipitation 11.5.1.2 Projections for SRES scenarios 670 change 649 11.5.2 Regional Sea Level Change 673 11.2.2.4 Evolution of area 650 11.5.3 Implications for Coastal Regions 674 11.2.3 Greenland and Antarctic Ice Sheets 650 11.5.3.1 Mean sea level 674 11.2.3.1 Mass balance studies 650 11.5.3.2 Extremes of sea level: storm- 11.2.3.2 Direct monitoring of surface surges and waves 675 elevation changes 652 11.5.4 Longer Term Changes 675 11.2.3.3 Numerical modelling 652 11.5.4.1 Thermal expansion 675 11.2.3.4 Sensitivity to climatic change 653 11.5.4.2 Glaciers and ice caps 677 11.2.4 Interaction of Ice Sheets, Sea Level and 11.5.4.3 Greenland and Antarctic ice the Solid Earth 654 sheets 677 11.2.4.1 Eustasy, isostasy and glacial- interglacial cycles 654 11.6 Reducing the Uncertainties in Future Estimates 11.2.4.2 Earth rotation constraints on of Sea Level Change 679 recent sea level rise 656 11.6.1 Observations of Current Rates of Global- 11.2.5 Surface and Ground Water Storage and averaged and Regional Sea Level Change 679 Permafrost 657 11.6.2 Ocean Processes 680 11.2.6 Tectonic Land Movements 658 11.6.3 Glaciers and Ice Caps 680 11.2.7 Atmospheric Pressure 659 11.6.4 Greenland and Antarctic Ice Sheets 680 11.6.5 Surface and Ground Water Storage 680 11.3 Past Sea Level Changes 659 11.6.6 Summary 681 11.3.1 Global Average Sea Level over the Last 6,000 Years 659 Appendix 11.1: Methods for Projections of Global- 11.3.2 Mean Sea Level Changes over the Past average Sea Level Rise 682 100 to 200 Years 661 11.3.2.1 Mean sea level trends 661 References 684 11.3.2.2 Long-term mean sea level accelerations 663 Changes in Sea Level 641 Executive Summary century, AOGCM simulations result in rates of thermal expansion of 0.3 to 0.7 mm/yr. This chapter assesses the current state of knowledge of the rate of change of global average and regional sea level in • The mass of the ocean, and thus sea level, changes as water is relation to climate change. We focus on the 20th and 21st exchanged with glaciers and ice caps. Observational and centuries. However, because of the slow response to past modelling studies of glaciers and ice caps indicate a contribu- conditions of the oceans and ice sheets and the consequent tion to sea level rise of 0.2 to 0.4 mm/yr averaged over the 20th land movements, we consider changes in sea level prior to the century. historical record, and we also look over a thousand years into the future. • Climate changes during the 20th century are estimated from modelling studies to have led to contributions of between –0.2 Past changes in sea level and 0.0 mm/yr from Antarctica (the results of increasing From recent analyses, our conclusions are as follows: precipitation) and 0.0 to 0.1 mm/yr from Greenland (from changes in both precipitation and runoff). • Since the Last Glacial Maximum about 20,000 years ago, sea level has risen by over 120 m at locations far from present and • Greenland and Antarctica have contributed 0.0 to 0.5 mm/yr former ice sheets, as a result of loss of mass from these ice over the 20th century as a result of long-term adjustment to past sheets. There was a rapid rise between 15,000 and 6,000 years climate changes. ago at an average rate of 10 mm/yr. • Changes in terrestrial storage of water over the period 1910 to • Based on geological data, global average sea level may have 1990 are estimated to have contributed from –1.1 to risen at an average rate of about 0.5 mm/yr over the last 6,000 +0.4 mm/yr of sea level rise. years and at an average rate of 0.1 to 0.2 mm/yr over the last 3,000 years. The sum of these components indicates a rate of eustatic sea level rise (corresponding to a change in ocean volume) from • Vertical land movements are still occurring today as a result of 1910 to 1990 ranging from –0.8 to 2.2 mm/yr, with a central these large transfers of mass from the ice sheets to the ocean. value of 0.7 mm/yr. The upper bound is close to the observational upper bound (2.0 mm/yr), but the central value is less than the • During the last 6,000 years, global average sea level variations observational lower bound (1.0 mm/yr), i.e., the sum of on time-scales of a few hundred years and longer are likely to components is biased low compared to the observational have been less than 0.3 to 0.5 m. estimates. The sum of components indicates an acceleration of only 0.2 mm/yr/century, with a range from –1.1 • Based on tide gauge data, the rate of global average sea level rise to +0.7 mm/yr/century, consistent with observational finding of during the 20th century is in the range 1.0 to 2.0 mm/yr, with a no acceleration in sea level rise during the 20th century. The central value of 1.5 mm/yr (as with other ranges of uncertainty, estimated rate of sea level rise from anthropogenic climate it is not implied that the central value is the best estimate). change from 1910 to 1990 (from modelling studies of thermal expansion, glaciers and ice sheets) ranges from 0.3 to 0.8 mm/yr. • Based on the few very long tide gauge records, the average It is very likely that 20th century warming has contributed signif- rate of sea level rise has been larger during the 20th century icantly to the observed sea level rise, through thermal expansion than the 19th century. of sea water and widespread loss of land ice. • No significant acceleration in the rate of sea level rise during Projected sea level changes from 1990 to 2100 the 20th century has been detected. Projections of components contributing to sea level change from 1990 to 2100 (this period is chosen for consistency with • There is decadal variability in extreme sea levels but no the IPCC Second Assessment Report), using a range of evidence of widespread increases in extremes other than that AOGCMs following the IS92a scenario (including the direct associated with a change in the mean. effect of sulphate aerosol emissions) give: Factors affecting present day sea level change • thermal expansion of 0.11 to 0.43 m, accelerating through the Global average sea level is affected by many factors. Our assess- 21st century; ment of the most important is as follows. • a glacier contribution of 0.01 to 0.23 m; • a Greenland contribution of –0.02 to 0.09 m; • Ocean thermal expansion leads to an increase in ocean volume • an Antarctic contribution of –0.17 to 0.02 m. at constant mass. Observational estimates of about 1 mm/yr over recent decades are similar to values of 0.7 to 1.1 mm/yr Including thawing of permafrost, deposition of sediment, and obtained from Atmosphere-Ocean General Circulation Models the ongoing contributions from ice sheets as a result of climate (AOGCMs) over a comparable period. Averaged over the 20th change since the Last Glacial Maximum, we obtain a range of 642 Changes in Sea Level global-average sea level rise from 0.11 to 0.77 m. This range Longer term changes reflects systematic uncertainties in modelling. If greenhouse gas concentrations were stabilised, sea level For the 35 SRES scenarios, we project a sea level rise of would nonetheless continue to rise for hundreds of years.
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